The first man-made plastic was invented in Britain in 1851 by Alexander Parkes. He publicly demonstrated it at the 1862 International Exhibition in London, calling the material Parkesine. Derived from cellulose, Parkesine could be heated, molded, and retain its shape when cooled. It was, however, expensive to produce, prone to cracking, and highly flammable. In 1868, American inventor John Wesley Hyatt developed a plastic material he named Celluloid, improving on Parkes' concept so that it could be processed into finished form. Hyatt patented the first injection molding machine in 1872. It worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mold. The industry expanded rapidly in the 1940s because World War II created a huge demand for inexpensive, mass-produced products. In 1946, American inventor James Watson Hendry built the first screw injection machine. This machine also allowed material to be mixed before injection, so that colored or recycled plastic could be added to virgin material and mixed thoroughly before being injected. In the 1970s, Hendry went on to develop the first gas-assisted injection molding process.
Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. They are also known as presses, they hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection process. Tonnage can vary from less than five tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The amount of total clamp force is determined by the projected area of the part being molded. This projected area is multiplied by a clamp force of from two to eight tons for each square inch of the projected areas. As a rule of thumb, four or five tons per square inch can be used for most products. If the plastic material is very stiff, more injection pressure may be needed to fill the mold, thus more clamp tonnage to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger parts require higher clamping force. With Injection Molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity through a gate and runner system. The mold remains cold so the plastic solidifies almost as soon as the mold is filled. Mold assembly or die are terms used to describe the tooling used to produce plastic parts in molding. The mold assembly is used in mass production where thousands of parts are produced. Molds are typically constructed from hardened steel, etc. Hot-runner systems are used in molding systems, along with mold assemblies, for the manufacture of plastic articles. Usually, hot-runners systems and mold assemblies are treated as tools that may be sold and supplied separately from molding systems.
U.S. Pat. No. 5,536,164 discloses a manifold assembly for supplying plastic material from a plastic source to a mold assembly in an injection molding machine and includes a flexible manifold having an interior conduit connected between the plastic source and the mold assembly. The flexible manifold is configured to define an input connector, a first curved segment attached to the input connector, a second curved segment, an output connector attaching the second curved segment to the mold assembly, and an intermediary segment connecting the first and second curved segments. This provides the flexible manifold with a generally S-shaped configuration that flexes with temperature changes to maintain a substantially constant positioning between the input connector and the output connector, preventing thermally induced movement of the mold assembly with respect to the input connector as heated plastic is injected through the conduit.
U.S. Pat. No. 5,738,149 discloses a manifold assembly for supplying plastic material from a plastic source to a mold assembly in an injection molding machine and includes a flexible manifold having an interior conduit connected between the plastic source and the mold assembly. The flexible manifold is configured to define an input connector, a first curved segment attached to the input connector, a second curved segment, an output connector attaching the second curved segment to the mold assembly, and an intermediary segment connecting the first and second curved segments. This provides the flexible manifold with a generally S-shaped configuration that flexes with temperature changes to maintain a substantially constant positioning between the input connector and the output connector, preventing thermally induced movement of the mold assembly with respect to the input connector as heated plastic is injected through the conduit.
U.S. Pat. No. 6,149,423 discloses a hot channel die that is arranged within a casing filled with oil. In the hollow space filled with oil, baffles are installed which effect a current of the oil directed toward the two ends of the die. For this purpose, the heating element is arranged on the underside of the casing. The discharge sleeve, which introduces the liquid plastic from the feeding screw, discharges into the hollow body, which is mounted in a recess in the hot runner plate of a hot channel injection molding die. The distribution conduits, which are constructed as curved tubes, are installed on the discharge sleeve, and lead to the side wall, against which the rear ends of the injection nozzles lie. The hollow space is filled with a heat-conducting medium, for example oil, which is heated by a heater and uniformly circulated within the hollow space by convection or motorized circulation. Baffles optimize the circulation of the medium and its return guidance to the heater.
U.S. Pat. No. 5,683,731 discloses a redistributing device for use with melt flow exhibiting boundary layer flow and centralized flow and comprises a body including a melt flow inlet end and a plurality of melt flow outlets. A first flow diverter is included for distributing at least the boundary layer flow among the plurality of the melt flow outlets. A second flow diverter is included for distributing at least the centralized flow among the plurality of melt flow outlets.
U.S. Pat. No. 4,965,028 discloses a method and apparatus for thermoplastic multigated single cavity or multicavity injection molding. A plasticized melt flows along a melt distributing passageway, and enters through a plurality of gates associated with one or more mold cavity. Melt temperature is maintained by means of manifold heaters, bushing heater band and, most preferably, heated probe. A unique melt conditioning element placed just upstream of gate forces the melt to enter a plurality of inlet melt channels and flow through a region of constricted cross section and/or angular change of flow direction formed by the geometric relationship of a bushing wall to said element. The result is to provide, by design, various degrees of melt heating, melt filtration, and melt homogenization.